754. Reach a Number

You are standing at position 0 on an infinite number line. There is a goal at position target.

On each move, you can either go left or right. During the n-th move (starting from 1), you take n steps.

Return the minimum number of steps required to reach the destination.

Example 1:

Input: target = 3
Output: 2
Explanation:
On the first move we step from 0 to 1.
On the second step we step from 1 to 3.

Example 2:

Input: target = 2
Output: 3
Explanation:
On the first move we step from 0 to 1.
On the second move we step  from 1 to -1.
On the third move we step from -1 to 2.

Note:

• target will be a non-zero integer in the range [-10^9, 10^9].
• 754. Reach a Number
struct Solution;

impl Solution {
fn reach_number(target: i32) -> i32 {
let target = target.abs();
let n = (((2 * target as i64) as f64 + 0.25).sqrt() - 0.5).ceil() as i32;
let sum = n * (n + 1) / 2;
if sum == target {
n
} else {
let diff = sum - target;
if diff % 2 == 0 {
n
} else {
if n % 2 == 0 {
n + 1
} else {
n + 2
}
}
}
}
}

#[test]
fn test() {
assert_eq!(Solution::reach_number(3), 2);
assert_eq!(Solution::reach_number(2), 3);
assert_eq!(Solution::reach_number(5), 5);
}